When preparing OLED devices usually printing techniques like inkjet printing, roll to roll printing, slot dye coating or gravure printing are used to apply the active layer. Based on low solubility of the most of the present organic compounds useful as emitting materials and/or charge transporting materials, these techniques need the use of solvents in high amounts.
Printing formulations usually comprise aromatic or aliphatic organic solvents and tend to have low viscosities. While this approach serves well for spin coating and ink jet printing (IJP) fabrication methods, recently there has been a growing interest in using traditional printing technologies, like flexographic or gravure printing, to fabricate devices. This requires different types of formulations, in particular with respect to the choice of the solvents and optional additives like viscosity enhancers.
In order to improve the film forming ability and the rheology binding agents can be used. These additives are especially needed with regard to light emitting materials and/or charge transporting materials having small molecular weight or polymeric compounds having a low molecular weight.
U.S. Pat. No. 6,541,300 discloses a process for preparing an organic semiconductor (OSC) film for use in an organic electronic (OE) device, by blending an OSC material with a multicomponent solvent blend and applying the resulting mixture to a substrate. It is claimed that the OSC film obtained by using a solvent blend has a higher mobility and the resulting OE device has a higher on/off ratio, compared to an OSC film, or an OE device including the OSC film, which are produced using only one solvent of the multicomponent solvent blend. However, the claimed process is characterized only by its desired result, i.e. a higher mobility and a higher on/off ratio, but not by the means how this result can be achieved. In particular, the document does not provide a clear guidance for the person skilled in the art how to select suitable solvents in order to achieve a workable formulation. The document only mentions that the solvents of the solvent blend should have a combined polarity (φm) from 0.1 to 1.0. Apart from this parameter, however, the document does not provide any limitation or guidance regarding the choice of the OSC compound, and the choice of solvents that are best suitable for a chosen OSC compound. Although the document discloses a list of OSC compounds (including polymers and small molecules) and a list of solvents having a polarity ranging from low to high values, it is not clear if all these solvents will readily dissolve the disclosed OSC compounds. However, it is known that when a solvent does not dissolve an OSC material, the resulting mixture is often not suitable for the preparation of OSC films and OE devices, since the morphology of the resulting OSC film will deteriorate, which will negatively affect the OE device performance. Therefore, in view of the teaching of U.S. Pat. No. 6,541,300, considerable effort is still required to find the appropriate solvents for a given OSC material as disclosed in this document.
WO 03/069959 A1 discloses an OSC film for use in an electroluminescent (EL) device, formed by a wet film process with a composite solution. The composite solution is prepared by dissolving at least two organic compounds in a mixed organic solvent, including at least two organic solvents having different volatility and different solubility for the organic compounds. Again, the document discloses a large list of possible solvents and organic compounds, but does not provide a clear guidance how suitable solvents can be chosen for a given organic compound. Instead, it is merely said that the selection of the at least two organic solvents having different volatility may depend on the property of the organic compounds.
EP 1 416 069 A1 discloses an OSC element comprising a polyacene as OSC material. The document further mentions that the polyacene may be dissolved in a solvent, and that a combination of two or more solvents may be used. However, apart from a list of standard solvents no preference is given to any particular solvent or solvent combination, and no specific guidance is provided regarding the selection of suitable solvents.
WO 2005/041322 A1 discloses an organic polymer solution for use in OE devices comprising at least one organic polymer, a first solvent and a second solvent, wherein the first solvent has a low solubility and faster evaporation rate than the second solvent, and the second solvent has a very low solubility. Also claimed is a method of manufacturing an OE device by depositing the solution on an electrode and allowing it to dry. It is claimed that, due to the different solubility and evaporation rate of the solvents, a substantially uniform polymer layer is formed. However, no specific values or parameter ranges of the solvent properties are given, which could serve as a basis for the selection of suitable solvents.
EP 1 122 793 A2 discloses an organic luminescence device manufactured from an ink comprising an organic EL material and a hydrophobic organic solvent or solvent mixture, wherein the solvents have a dissolving power of at most 5 wt. % of water at room temperature. However, this value does apply to practically all OLED solvents known in the prior art and does therefore not constitute a real limitation. It is further mentioned that the ink should have a viscosity of not more than 5000 cp, preferably not more than 100 cp. However, there is no guidance how to select specific solvents to achieve these values. This makes the selection of suitable solvents still difficult, especially when trying to prepare dilute small molecule solutions having high viscosities, without using non-volatile thickening agents.
WO 03/053707 A2 discloses a screen printable EL polymer ink comprising a soluble EL material, an organic solvent having a boiling point between 120 and 200° C., and a viscosity enhancer to maintain a viscosity of above 50 cp. The organic solvent should preferably have a solubility parameter of between 8.8 and 10.0 (cal/cm3)1/2. The majority of viscosity modifiers mentioned in this document are polymers or inorganic particles, as for example disclosed on page 9 or 10. Further reported is the use of “gel retarders” in concentrations from 1 to 20%, which can also be commercially available products, to decrease solvent evaporation and improve ink stability and processability. However, the use of processing additives as suggested in this document is not always desirable in OSC printing inks for the preparation of OE devices, since these processing additives will remain in the OSC layer after removal of the solvent, where they could negatively affect or even destroy the performance of the device. Instead it is more preferable to have only the pure active OSC material left in the OSC layer after drying, without any processing additives. Therefore, apart of the active OSC material the ink should preferably contain only volatile components.
Solutions of small molecules in a solvent do not generally have a big effect on the viscosity of the resultant solution as occurs with polymers. Therefore, compositions for conventional printing applications like ink jet printing, flexographic or gravure printing need additives to increase ink viscosity and improve film formation. The prior art provides compositions being useful in order to process low molecular weight organic light emitting and charge transporting materials. However, it is a permanent desire to improve the performance of the OLED layer, such as efficiency, lifetime and sensitivity regarding oxidation or water.
It is therefore desirable to have fluids comprising light emitting materials and/or charge transporting materials that are suitable for the preparation of OLED devices, which allow the manufacture of high efficient OLED devices having a high performance, a long lifetime and a low sensitivity against water or oxidation. One aim of the present invention is to provide such improved fluids. Another aim is to provide improved methods of preparing an OLED device from such fluids. Another aim is to provide improved OLED devices obtained from such fluids and methods. Further aims are immediately evident to the person skilled in the art from the following description.
Surprisingly it has been found that these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OLED device using a composition of the present invention.